The present invention relates in general to signal processing for industrial control applications, and in particular to a new and useful input circuit for receiving a wide variety of signals.
Industrial digital and frequency inputs normally are a resistive circuit with opto-coupling for the required isolation. They are able to take a standard control signal such as 24 VDC, 125 VDC, 117 VAC, or 230 VAC.
A drawback to these circuits is that they require jumper settings to select a desired signal range. Otherwise, the circuits are limited to only one signal type. A common problem is that customers end up with the jumpers in the wrong positions for their application, thus failure occurs.
Another problem with a resistive input is that as the applied voltage is increased, the power consumption increases at an accelerated rate. Also, there is a limit to signal knowledge depending on the method of monitoring. For example, if the level is checked ten times a second, it may be adequate for a digital input; however, a 5 Hz frequency is the highest it would be able to pick up according to Shannon's sampling theorem. On the other hand, if it is monitored at smaller increments like 20,000 times a second, then there probably would not be much time to do anything else. At this point a frequency-to-voltage converter may seem appealing; however, this requires an analog signal measurement which is indirect and more difficult to implement.